1. Field of the Invention
The present invention relates to a surface acoustic wave device, a method for producing the same, and a circuit module using the same.
2. Description of the Related Art
Recently, communication equipment such as a mobile phone is being miniaturized and reduced in size rapidly. Along with this, there is a demand for miniaturization of a surface acoustic wave device (hereinafter, which may be referred to as a xe2x80x9cSAW devicexe2x80x9d) such as a filter and a resonator mounted on communication equipment. There also is a demand for miniaturization of the space required for setting a SAW device.
FIG. 38 schematically shows a cross-sectional view (hatching is omitted) of a SAW device 900 as a representative example of a conventional SAW device. The SAW device 900 includes a piezoelectric substrate 901, comb electrodes (alternatively called inter-digital transducers) 902 and electrode pads 903 formed on the piezoelectric substrate 901, wires 904, electrode pads 905, internal electrodes 906, external electrodes 907, a layered ceramic substrate 908 composed of a layered body of ceramic substrates 908a, 908b, and 908c, and a cover portion 909.
The comb electrodes 902 excite a surface acoustic wave. The comb electrodes 902 are connected electrically to the electrode pads 903 via wiring lines (not shown) formed on the piezoelectric substrate 901. The comb electrodes 902 are connected electrically to the external electrodes 907 via the electrode pads 903, the wires 904, the electrode pads 905, and the internal electrodes 906. In the SAW device, it is required to form an enclosed space around the comb electrodes 902 for the purpose of ensuring propagation of a surface acoustic wave. In the SAW device 900, the layered ceramic substrate 908 and the cover portion 909 form an enclosed space.
However, in the SAW device 900, it is required to form wires 904 three-dimensionally, and enlarge the electrode pads 903 and 905 for wire bonding. Therefore, such a configuration of the SAW device 900 hinders miniaturization thereof. Furthermore, in the SAW device 900, a parasitic inductance caused by the wires 904 is large.
In order to solve the above-mentioned problems, a method for mounting a SAW element provided with the piezoelectric substrate 901, the comb electrodes 902, and the electrode pads 903 onto a substrate by a facedown technique has been reported (see JP 5(1993)-55303 A). FIG. 39 schematically shows a cross-sectional view (partial hatching is omitted) of a SAW device 950 as an example of such a SAW device. The SAW device 950 includes a piezoelectric substrate 901, comb electrodes 902 and electrode pads 903 formed on the piezoelectric substrate 901, bumps 951, electrode pads 952, internal electrodes 953, external electrodes 954, a substrate 955, dams 956, and a resin film 957.
The comb electrodes 902 are connected electrically to the external electrodes 954 via the electrode pads 903, the bumps 951, the electrode pads 952, and the internal electrodes 953. On the periphery of the comb electrodes 902, an enclosed space is provided by the resin film 957 formed so as to cover the piezoelectric substrate 901. The dams 956 prevent resin from flowing into the enclosed space when the resin film 957 is formed. In the SAW device 950, a SAW element is mounted on the substrate 955 by a face-down technique, so that the SAW device in this example is more likely to be miniaturized compared with the SAW device 900.
There also is a method for mounting a SAW element in an air-tight container of the SAW device 900 by a face-down technique.
However, the SAW device 905 has the following problems. In order to produce the SAW device 950, it is required to form the electrode pads 952, the internal electrodes 953, and the external electrodes 954 on the surface of and inside the substrate 955. During this process, if the substrate 955 is thin, the substrate 955 may be warped or damaged. This makes it difficult to make the substrate 955 thin, resulting in insufficient miniaturization of an apparatus.
Furthermore, in the course of producing the SAW device 950, the dams 956 prevent the resin to be the resin film 957 from flowing into the enclosed space. This requires that mounting is conducted while the interval between the piezoelectric substrate 901 and the substrate 955 is regulated with high precision. If the bumps 951, the electrode pads 952, and the internal electrodes 953 are mounted so as to be positioned in a line, it is difficult to conduct mounting with high precision. This is because the material for the internal electrode 953 is different from that for the substrate 955. Therefore, in the SAW device 950, the bumps 951 and the internal electrodes 953 are disposed so as to be shifted from each other, which makes it difficult to achieve sufficient miniaturization.
Furthermore, with a device in which a SAW element is mounted in an air-tight container of the SAW device 900 by a face-down technique, an airtight container capable of accommodating the SAW device is required. Therefore, even with such a device, sufficient miniaturization is difficult.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a SAW device capable of being further miniaturized compared with a conventional SAW device, and a method for producing the same.
In order to achieve the above-mentioned object, a surface acoustic wave device of the present invention includes: a piezoelectric substrate; a plurality of comb electrodes for exciting a surface acoustic wave, disposed on a principal plane of the piezoelectric substrate; a plurality of bumps disposed on the principal plane; and a member containing resin, disposed on the principal plane side, wherein the bumps and the comb electrodes are connected electrically to each other, and at least a part of the bumps is buried in the member. The surface acoustic wave device of the present invention can be used, for example, for a frequency filter and a resonator mounted in communication equipment.
In the above-mentioned surface acoustic wave device, the member may be an insulating sheet, the insulating sheet may be disposed away from the comb electrodes, and the bumps may penetrate through the insulating sheet.
The above-mentioned surface acoustic wave device further includes a circuit board disposed so as to be opposed to the principal plane of the piezoelectric substrate, wherein the circuit board includes wiring lines formed on a surface on the piezoelectric substrate side, the member is disposed between the piezoelectric substrate and the circuit board, and the member is a space forming member for forming a space that allows the comb electrodes to vibrate on a periphery of the comb electrodes.
Furthermore, a first method for producing a surface acoustic wave device of the present invention includes the steps of: (a) forming a plurality of comb electrodes for exciting a surface acoustic wave and bumps connected electrically to the comb electrodes on a principal plane of a piezoelectric substrate; and (b) opposing the principal plane of the piezoelectric substrate to an insulating sheet, and bringing the piezoelectric substrate and the insulating sheet closer together so that the bumps penetrate through the insulating sheet.
Furthermore, a second method for producing a surface acoustic wave device of the present invention includes the steps of: (i) forming a piezoelectric substrate, a plurality of comb electrodes formed on a principal plane of the piezoelectric substrate, a surface acoustic wave element including a plurality of bumps connected electrically to the plurality of comb electrodes, and a circuit board having a principal plane on which wiring lines are formed; (ii) disposing the principal plane of the piezoelectric substrate and the principal plane of the circuit board so that they are opposed to each other with a space forming member interposed therebetween; and (iii) bringing the circuit board and the surface acoustic wave element closer together so that the bumps are buried in the space forming member to be connected electrically to the wiring lines. In the step (iii), the space forming member forms a space allowing the comb electrodes to vibrate on a periphery of the comb electrodes.
Furthermore, a circuit module of the present invention includes a circuit board, a surface acoustic wave device, and a functional element, wherein the circuit board includes wiring lines formed on a principal plane thereof, the surface acoustic wave device and the functional element are mounted on the wiring lines, the surface acoustic wave device includes a piezoelectric substrate, a plurality of comb electrodes for exciting a surface acoustic wave disposed on a principal plane of the piezoelectric substrate, a plurality of bumps disposed on the principal plane of the piezoelectric substrate, and a member containing resin disposed on a side of the principal plane of the piezoelectric substrate, wherein the bumps and the comb electrodes are connected electrically to each other, and at least a part of the bumps is buried in the member.
In a circuit module of the present invention, the member may be an insulating sheet, the insulating sheet may be disposed away from the comb electrodes, and the bumps may penetrate through the insulating sheet.
In a circuit module of the present invention, the member may be disposed between the piezoelectric substrate and the circuit board, and the member may be a space forming member for forming a space that allows the comb electrodes to vibrate on a periphery of the comb electrodes.
The SAW device of the present invention can be further miniaturized compared with a conventional SAW device, and reduction in cost and enhancement of reliability can be realized. The SAW device of the present invention can be used, for example, for a frequency filter and a resonator mounted in communication equipment.
Furthermore, according to the production method of a SAW device of the present invention, the SAW device of the present invention can be produced easily.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.